Igneous Rock Types

Why This Matters

Igneous rocks are where the rock cycle begins. Understanding how they form connects directly to plate tectonics, volcanic activity, and crustal composition. On exams, you'll need to connect a rock's texture to its cooling history, or predict what kind of igneous rock forms at a specific tectonic setting. Think of it as reading a rock like a story: texture tells you cooling rate, composition tells you magma chemistry, and location tells you tectonic context.

Don't just memorize rock names and colors. Know why granite has large crystals while basalt has tiny ones. Understand how silica content controls viscosity and eruption style. When you can explain the mechanism behind each rock type, you'll handle both multiple-choice questions and free-response prompts that ask you to compare formation environments or predict rock properties.

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Intrusive (Plutonic) Rocks: Slow Cooling Below the Surface

When magma cools slowly deep within Earth's crust, crystals have time to grow large and visible. The slower the cooling, the larger the crystals. That single principle explains all coarse-grained igneous rocks.

Granite

• Coarse-grained texture with visible crystals of quartz, feldspar, and mica. This is the textbook example of slow cooling in a plutonic environment.
• Felsic composition means it's rich in silica (roughly 70%+) and light in color. High silica also means the original magma was very viscous.
• Continental crust foundation. Granite batholiths form the cores of mountain ranges and are extremely resistant to weathering.

Gabbro

• Coarse-grained and dark-colored, composed mainly of plagioclase feldspar and pyroxene. It's the intrusive equivalent of basalt (same chemistry, different texture).
• Mafic composition with high iron and magnesium content gives it greater density than granite.
• Makes up the lower oceanic crust, sitting beneath the basalt layer. Knowing this pairing is critical for understanding seafloor structure.

Diorite

• Intermediate composition between granite and gabbro, often displaying a distinctive salt-and-pepper speckled appearance.
• Contains plagioclase feldspar and hornblende. The mix of light and dark minerals reflects its chemical middle ground.
• Forms in continental crust from slowly cooling magma, often found alongside granite in plutonic complexes.

Pegmatite

• Exceptionally large crystals, some measuring meters across, formed during the final stages of magma crystallization.
• Water-rich residual magma allows ions to move freely through the melt, producing giant crystal growth typically along the margins of granite bodies.
• Source of rare minerals including lithium, beryllium, and gemstones like tourmaline and aquamarine.

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Extrusive (Volcanic) Rocks: Rapid Cooling at the Surface

When lava erupts and cools quickly at Earth's surface, crystals have little time to form. Rapid cooling produces fine-grained textures or even glass. This is the opposite of what happens underground.

Basalt

• Fine-grained, dark-colored, and mafic. It forms from low-viscosity lava that flows easily across the surface.
• Dominates the ocean floor and builds shield volcanoes (like Hawaii), volcanic islands, and flood basalt provinces.
• Extrusive equivalent of gabbro. Same composition, completely different texture because of cooling rate.

Rhyolite

• Fine-grained and felsic, rich in silica with quartz and feldspar. It's the extrusive equivalent of granite.
• High-viscosity lava traps gases, which leads to explosive eruptions rather than gentle flows.
• Associated with continental volcanism and caldera-forming events like those at Yellowstone.

Andesite

• Intermediate composition between basalt and rhyolite, typically gray to brown.
• Subduction zone signature. Andesite is commonly found in volcanic arcs where oceanic crust dives beneath continental crust. The Andes Mountains (its namesake) are a classic example.
• Often porphyritic, with larger crystals embedded in a fine-grained matrix. This texture records two-stage cooling: slow crystallization at depth, then rapid cooling after eruption.

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Volcanic Glass and Vesicular Textures: Extreme Cooling Conditions

Some volcanic rocks form under such rapid or gas-rich conditions that they develop unique textures: glassy surfaces from instant cooling or vesicles (holes) from trapped gas bubbles escaping during solidification.

Obsidian

• Volcanic glass formed when silica-rich lava cools so rapidly that no crystals can form at all.
• Conchoidal fracture produces razor-sharp edges, which is why obsidian was historically valuable for tools and weapons.
• Felsic composition despite its dark color. The glassy texture is what makes it look dark; if it had time to crystallize, it would resemble light-colored rhyolite or granite.

Pumice

• Highly vesicular and low-density. Pumice is the only common rock that floats on water, thanks to abundant trapped gas bubbles.
• Forms during explosive felsic eruptions when frothy, gas-charged magma rapidly depressurizes and solidifies mid-eruption.
• Used as an abrasive in industrial and cosmetic applications because of its rough, porous texture.

Scoria

• Vesicular and mafic, typically reddish-brown or black. Think of it as the basaltic counterpart to pumice.
• Denser than pumice because mafic lava generally has lower gas content, producing larger but less numerous vesicles.
• Common in cinder cones and often used as lightweight landscaping aggregate.

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Quick Reference Table

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Self-Check Questions

1. Texture comparison: Granite and basalt can share similar compositions with their intrusive/extrusive counterparts. What single factor explains why granite is coarse-grained and basalt is fine-grained?

2. Identify by concept: Which two rocks would you expect to find at a mid-ocean ridge, and what roles do they play in oceanic crust structure?

3. Compare and contrast: How do pumice and scoria demonstrate the relationship between magma composition, gas content, and rock density?

4. Tectonic connection: If you found andesite with porphyritic texture at a volcanic arc, what does this tell you about both the tectonic setting and the cooling history of the magma?

5. FRQ practice: Explain why rhyolite and granite have the same chemical composition but form under different conditions. How would you use these two rocks to illustrate the difference between intrusive and extrusive igneous processes?